1. Field of the Invention
The present invention generally relates to a constant voltage regulator, and more particularly to a constant voltage regulator that generates a relatively low voltage output and is capable of suppressing a leak current from an output transistor.
2. Discussion of the Related Art
As one example of a background constant voltage regulator, FIG. 1 illustrates a background constant voltage regulator 100 which includes a reference voltage generator 101, an error amplifier 102, an output transistor M101, and resistors R1 and R2. The output transistor M101 includes a PMOS (p-channel metal oxide semiconductor) transistor. The output transistor M101 receives a control signal at its gate terminal so as to convert an input voltage Vin to an output voltage Vout to be supplied to a load 104 (e.g., an external host apparatus) through an output terminal OUT. The resistors R1 and R2 are connected in series between the output terminal OUT and a point of a ground voltage level. The error amplifier 102 amplifies a voltage difference between a predetermined reference voltage Vr generated by the reference voltage generator 101 and a divided voltage Vfb produced at a connection point between the resistors R1 and R2 based on the output voltage Vout. The amplified voltage difference is output as the control signal from the error amplifier 102 to the gate terminal of the output transistor M101 so that the operations of the output transistor M101 is controlled. The input voltage Vin is generated by an external power source 103 is supplied to the output transistor M101 through an input terminal IN of the constant voltage regulator 100. The input voltage Vin is ordinary supplied also to a substrate gate of the output transistor M101.
To address low power consumption of the load 104, the output voltage Vout is generally desired to be small. As a consequence, it is not uncommon to set the output voltage Vout to a considerably small voltage, even less than 1 volt, for example. Also, to address a high efficiency of the constant voltage regulator 100, the input voltage Vin and the output voltage Vout consequently have a small difference in voltage. Therefore, the output transistor M101 needs to have a relatively low threshold voltage Vth. It is generally known that a MOS (metal oxide semiconductor) transistor having a relatively low threshold voltage adversely increases a leak current when turned off.
If the output transistor M101 has a relatively large leak current, it may happen that the leak current exceeds a consumption current when the consumption current becomes extremely small in such a case where the load 104 is in a standby mode. In this case, the output voltage Vout may be increased to a value above a rated voltage.
Also, if the output transistor M101 has a relatively large leak current, the leak current flows through the load 104 and, therefore, some voltage is produced and appears at the output terminal OUT when the operations of the error amplifier 102 are stopped so as to turn off generating the output voltage Vout. As a result, the constant voltage regulator 100 cannot be turned off.
One known attempt to reduce the leak current generated by the PMOS transistor of the output transistor M101 is to apply as large voltage as possible to the substrate gate of the PMOS transistor of the output transistor M101. One example circuit of a constant voltage regulator 100a having such a structure is shown in FIG. 2. The constant voltage regulator 100a of FIG. 2 is similar to the constant voltage regulator 100 of FIG. 1, except for an addition of a voltage selection circuit 110. As illustrated in FIG. 2, the voltage selection circuit 110 includes a diode D111, a PMOS transistor M111, a comparator 111, a reference voltage generator 112, a current source 113, a PNP transistor Q111, and a direct-current source 114.
In the voltage selection circuit 110, the current source 113 and the PNP transistor Q111 form a temperature detector. At a regular temperature, the PMOS transistor M11 is turned off and the input voltage Vin is applied to the substrate gate of the output transistor M101 via the diode D111. A base-emitter voltage of the PNP transistor Q111 is lowered with an increase of temperature. When the base-emitter voltage of the PNP transistor Q111 is lowered below an output voltage Vrl of the reference voltage generator 112, an output signal of the comparator 111 falls to a low level. As a consequence, the PMOS transistor M111 is turned on, and a voltage Vs greater than the input voltage Vin is supplied from the direct-current source 114 to the substrate gate of the output transistor M101. In this way, the constant voltage regulator 100a addresses a problem of an increase leak current of the output transistor M101 at a high temperature.
However, the constant voltage regulator 100a produces another problem. When a voltage supplied to the substrate gate of the MOS transistor is increased, the MPS transistor may increase its threshold voltage Vth. That is, it is a dilemma. To address a low power consumption, the output transistor needs a small threshold voltage but produces a leak current instead. Then, to address a reduction of the leak current, the output transistor increases the voltage of the substrate gate which increases the threshold voltage.